The present invention relates generally to printing mechanisms, such as inkjet printers or inkjet plotters. Printing mechanisms often include an inkjet printhead which is capable of forming an image on many different types of media. The inkjet printhead ejects droplets of colored ink through a plurality of orifices and onto a given media as the media is advanced through a printzone. The printzone is defined by the plane created by the printhead orifices and any scanning or reciprocating movement the printhead may have back-and-forth and perpendicular to the movement of the media. Methods for expelling ink from the printhead orifices, or nozzles, include piezo-electric and thermal techniques which are well-known to those skilled in the art. For instance, two earlier thermal ink ejection mechanisms are shown in U.S. Pat. Nos. 5,278,584 and 4,683,481, both assigned to the present assignee, the Hewlett-Packard Company.
In a thermal inkjet system, a barrier layer containing ink channels and vaporization chambers is located between a nozzle orifice plate and a substrate layer. This substrate layer typically contains columnar arrays of heater elements, such as resistors, which are individually addressable and energized to heat ink within the vaporization chambers. Upon heating, an ink droplet is ejected from a nozzle associated with the energized resistor. The inkjet printhead nozzles are typically aligned in one or more columnar arrays substantially parallel to the motion of the print media as the media travels through the printzone.
Typically, the print media is advanced under the inkjet printhead and held stationary while the printhead passes along the width of the media, firing its nozzles as determined by a controller to form a desired image on an individual swath, or pass. The print media is usually advanced between passes of the reciprocating inkjet printhead in order to avoid uncertainty in the placement of the fired ink droplets.
A printing mechanism may have one or more inkjet printheads, corresponding to one or more colors, or xe2x80x9cprocess colorsxe2x80x9d as they are referred to in the art. For example, a typical inkjet printing system may have a single printhead with only black ink; or the system may have four printheads, one each with black, cyan, magenta, and yellow inks; or the system may have three printheads, one each with cyan, magenta, and yellow inks. Of course, there are many more combinations and quantities of possible printheads in inkjet printing systems, including seven and eight ink/printhead systems.
Advanced printhead designs now permit an increased number of nozzles to be implemented on a single printhead. Thus, whether a single reciprocating printhead, multiple reciprocating printheads, or a page-wide printhead array are present in a given printing mechanism, the number of ink droplets which can be ejected per second is increased. While this increase in firing rate and density allows faster printing speeds, or throughput, there is also a corresponding increase in the amount of firing data which may be communicated from the printing mechanism controller to the printhead or printheads. In order to accommodate the faster data rates while reducing the conducted or radiated electromagnetic interference (EMI), constant current differential signaling techniques, such as low-voltage differential signaling (LVDS), have been implemented to transfer data from a controller to a printhead in printing mechanisms. An example of such an LVDS system is disclosed in commonly-owned, co-pending U.S. application Ser. No. 09/779,281.
Printing mechanisms may include LVDS drivers which receive firing signals from the controller and process the firing signals into a corresponding set of LVDS signals. The LVDS driver contains a constant current source which limits the output current to approximately three milliamps, while a switch steers the current between two transmission lines terminated by a resistor. This differential driver produces odd-mode transmission, where equal and opposite currents flow in the transmission lines. An LVDS driver produces no spike currents, and data rates as high as 1.5 gigabits per second are possible. Additionally, the constant current LVDS driver can tolerate the transmission lines being shorted together or to ground without creating thermal problems. This is advantageous, since ink shorting from the highly conductive ink residue and aerosol is a concern in inkjet printing mechanisms. Ink residue may build up on the printhead nozzle surface and migrate onto the printhead connector pads through normal printer operation or removal and installation of the printheads themselves. Similarly, air-borne aerosol may deposit onto the printhead contacts, creating a potential shorting situation for the LVDS transmission lines.
Unfortunately, despite the LVDS driver""s tolerance for transmission lines shorted to each other, the LVDS driver and associated controller electronics, as well as the replaceable printhead may easily be damaged by an ink short to a DC power line. Relatively high DC voltages are received by the printhead to heat the resistors in the vaporization chambers of the printhead and thereby cause ink to be ejected from printhead nozzles. The ink residue and aerosol which are capable of shorting LVDS transmission lines together are also capable of shorting the LVDS transmission lines to the DC voltage, thereby resulting in a catastrophic failure of the printing mechanism components.
Prior printing mechanisms have used diodes to disallow the transmission lines from exceeding a maximum voltage in the event that an ink short occurred. This solution, however, is no longer viable with high-speed signaling as a result of the excessive capacitance a power diode presents to a weakly driven LVDS signal. Thus, shunt and zener diodes are not desirable for use as short protection with an LVDS system. Therefore, it would be desirable to have a robust and inexpensive system for protecting constant current differential signaling printer drivers, such as LVDS drivers, and printer electronics from the devastating effects of power supply currents in the event of ink shorts.